1. Field of Use
This invention relates generally to methods and apparatus for manufacturing metal rods used in pellet sizing screens. In particular, it relates to methods and apparatus for applying chromium plating to elongated stainless steel and carbon steel rod cores of regular (circular) or irregular (T-shaped) cross sectional configuration.
2. Description of the Prior Art
Low grade iron ore such as taconite is processed by crushing to fine powder, magnetically separating the magnetite from unwanted silica, mixing the magnetite with a clay, such as bentonite, and water, feeding the resultant slurry to a hollow rotatable balling drum which forms the slurry into generally spherical green or unfired pellets and then discharging the pellets from the drum onto a pellet sizing screen which separates undersized ore pellets from those of a generally uniform desired size. The sizing screen, which may take any of several known forms, comprises a plurality of laterally spaced apart elongated metal screen rods between which undersized pellets drop onto a conveyor for reprocessing, whereas pellets of desired size are advanced along the screen for further processing.
Vibrating screens, for example, typically comprise rods of circular cross section. Trommel or rotating screens typically comprise arcuately shaped profile rods of T-shaped cross section which are arranged in rings and disposed in a coaxial conical arrangement at the end of the balling drum and are rotatable therewith. Roll screens comprise a plurality of laterally spaced apart rotatable rods of circular cross section serving as rollers onto which pellets are discharged from the balling drum.
The pellets being screened are highly abrasive and tend to mechanically wear down and break the metal rods forming the screen. Furthermore, since the magnetite in the pellets has magnetic properties, screen rods fabricated of ferromagnetic material tend to attract pellet particles and other magnetizable debris which adheres magnetically to the rods which interferes with the spacing between rods and affects screen efficiency. Replacement of worn or broken rods and cleaning of clogged screens requires shutdown of the apparatus, or some other mechanical means, and is costly and time consuming.
Stainless steel rods having non-magnetic properties overcome the aforementioned magnetic problems but are relatively expensive initially and are difficult to repair by welding when worn or broken. On the other hand, rods formed of non-magnetic metals other than stainless steel are much more prone to abrasive wear than rods formed of stainless steel or ferromagnetic materials such as carbon steel.
U.S. Pat. No. 3,848,744 suggests that screen rods be fabricated of a metal inner core having appropriate fatigue resistance, cost and workability and comprising a material such as plain carbon steel or other ferromagnetic material or non-ferrous, non-magnetic material such as nickel, aluminum, stainless steel or copper alloys. That patent further suggests that the inner core be provided with an attached outer shell having a high degree of abrasion resistance and paramagnetic properties and consisting of a plating of hard chromium, or tungsten carbide, or pyrolite carbon or molybdenum, which outer shell could be applied by standard electrolytic processes, fusion at high temperatures, sintering, or application through a plasma arc.